The present invention relates to an optical isolator having a function of allowing signal light for optical communication to pass therethrough but not allowing return light due to reflection to pass therethrough, and particularly to a polarization-independent optical isolator capable of exhibiting its function for signal light irrespective of the polarization plane of the signal light.
In the case of long-distance transmission of light by way of fiber optics communication or in the case of branching light transmitted via an optical fiber into light parts, when the quantity of light is attenuated, the light is amplified by an erbium doped fiber amplifier (EDFA) or the like. The optical amplifier, which is intended to directly amplify signal light without converting a light signal into an electric signal, is composed of a number of optical components such as a lens, a mirror, a filter and the like. Accordingly, return light reflected from these optical components and/or return light from the optical fiber end occur in the optical amplifier, to generate resonance of light in the optical amplifier, which leads to deterioration of the amplifying characteristic. To cope with such an inconvenience, an optical isolator for cutting off return light due to reflection is used. On the other hand, since the polarization state of signal light passing through an optical fiber is not constant because of the effect of an external stress and a bending force applied to the optical fiber, the optical isolator may be desirable to be of a polarization-independent type which is not dependent on the polarization state of signal light.
The polarization-independent optical isolator includes, as described in Japanese Patent Publication No. Sho 61-58809, two birefringent wedge plates and one Faraday rotator element. Such a polarization-independent optical isolator has a high isolation characteristic and is suitable for miniaturization thereof.
For the purpose of attaining a higher isolation characteristic and reducing a polarization mode dispersion (PMD), a polarization-independent optical isolator having a configuration that two optical isolator units are arranged in series wherein the optical axis of the final stage birefringent plate of the first unit is perpendicular to the optical axis of the first stage birefringent plate of the second unit has been disclosed, for example, in Japanese Patent No. 2,747,775.
The above-described prior art optical isolator is assembled by a method including steps of joining optical elements to each other by using metal solder or low melting point glass, and a number of intermediate steps such as angle adjustment, which are inserted in the above joining steps. As a result, the assembly of the prior art optical isolator is very complicated and takes a lot of time and labor. A method of realizing reduction of cost, size, and the number of steps of an optical isolator has been disclosed, for example, in Japanese Patent Laid-open No. Hei 9-138372. Such a method involves preparing birefringent parallel flat plates each of which is long enough to be easily processed, more specifically, has a length equivalent to lengths of optical elements of a plurality of optical isolators; sticking optical planes of these birefringent plates to each other with an adhesive or the like; and cutting the birefringent plates thus integrated into chips of the optical isolators.
Meanwhile, the polarization-independent optical isolator using birefringent wedge plates is excellent in essential characteristics such as isolation characteristic; however, it is unsuitable for mass-production and becomes inexpensive because of special shapes of the wedge plates.
A two-stage optical isolator using birefringent taper plates has been disclosed, for example, in Japanese Patent No. 1393843, in which an optical isolator unit including a first 45xc2x0 Faraday rotator disposed between first and second birefringent taper plates and an optical isolator unit including a second 45xc2x0 Faraday rotator disposed between third and fourth birefringent taper plates are arranged in series, wherein the inclination direction of the first and second birefringent taper plates is offset 90xc2x0 from the inclination direction of the third and fourth birefringent taper plates. Such a two-stage optical isolator is effective to give a high isolation characteristic. Japanese Patent Laid-open No. Hei 1-105908 has disclosed an optical isolator in which an optical isolator unit including a first 45xc2x0 Faraday rotator disposed between first and second polarization isolation elements (including birefringent taper plates) and an optical isolator unit including a second 45xc2x0 Faraday rotator disposed between third and fourth polarization isolation elements (including birefringent taper plates) are arranged in series, wherein the optical axis of the second polarization isolation element is offset 90xc2x0 from the optical axis of the third polarization isolation element. Such an optical isolator is effective to give a significantly higher isolation characteristic, and the like. Further, an optical isolator intended to improve the PMD characteristic by combining the configurations of the above-described two kinds of the optical isolators with each other has been disclosed in Japanese Patent Publication No. 2747775.
The above-described two-stage optical isolator, however, has a disadvantage. Namely, the inclination direction of the birefringent taper plates of the first optical isolator unit is offset 90xc2x0 from the inclination direction of the birefringent taper plates of the second optical isolator unit for ensuring the highest isolation characteristic; however, since compensation plates or the like are not used, an isolation distance between normal and abnormal light components of signal light in the forward direction becomes large, and accordingly, the signal light, which has been collimated after emerged from the optical fiber and has passed through the optical isolator, is restricted again by a lens, with a result that excess loss occurs when the signal light enters the optical fiber.
A first object of the present invention is to provide a polarization-independent optical isolator capable of enhancing the mass-production characteristic and realizing the cost reduction, and to provide a method of producing the polarization-independent optical isolator.
A second object of the present invention is to provide an optical isolator capable of enhancing the isolation characteristic, improving the PMD characteristic, and reducing the excess loss.
To achieve the first object, according to a first aspect of the present invention, there is provided a polarization-independent optical isolator including: a first birefringent wedge plate having an inclined plane directed to a light incoming side; a second birefringent wedge plate having an inclined plane directed to a light outgoing side, the wedge equal thickness line of the inclined plane of the second birefringent wedge plate being in parallel to the wedge equal thickness line of the inclined plane of the first birefringent wedge plate; and a Faraday rotator element disposed between the first and second birefringent wedge plates; wherein a non-inclined plane of the first birefringent wedge plate is adhesively bonded to a light incoming plane of the Faraday rotator element, and a light outgoing plane of the Faraday rotator element is adhesively bonded to a non-inclined plane of the second birefringent wedge plate; and the first and second birefringent wedge plates and the Faraday rotator element thus adhesively bonded to each other are disposed in a cylindrical magnet.
With this configuration, the light incoming plane and the light outgoing plane of the first birefringent wedge plate are formed of the inclined plane and non-inclined plane, respectively while the light incoming plane and the light outgoing plane of the second birefringent wedge plate are formed of the non-inclined plane and the inclined plane, respectively; the first and second birefringent wedge plates are arranged with their inclined planes directed outwardly; and the Faraday rotator element is held and adhesively bonded between the non-inclined planes of the first and second birefringent wedge plates. Accordingly, it is possible to solve the difficulty in assembly due to the wedge shaped parts, and to realize a significantly small-sized, easy-to-assemble structure without impairing the essential characteristics as the optical isolator.
In the above polarization-indepedent optical isolator, preferably, the cylindrical magnet is composed of two semi-cylindrical magnets each having an inner recess; and the first birefringent wedge plate, the Faraday rotator element, and the second birefringent wedge plate are fitted in and adhesively bonded to the two inner recesses of the two semi-cylindrical magnets, and simultaneously the two semi-cylindrical magnets are adhesively bonded to each other.
The first birefringent wedge plate and/or the second birefringent wedge plate are preferably made from a crystal of lithium niobate.
To achieve the first aspect, according to a second aspect of the present invention, there is provided a multi-stage polarization-independent optical isolator including: an even number of optical isolators assembled to each other, each of the optical isolators including a first birefringent wedge plate having an inclined plane directed to a light incoming side; a second birefringent wedge plate having an inclined plane directed to a light outgoing side, the wedge equal thickness line of the inclined plane of the second birefringent wedge plate being in parallel to the wedge equal thickness line of the inclined plane of the first birefringent wedge plate; and a Faraday rotator element disposed between the first and second birefringent wedge plates; wherein a non-inclined plane of the first birefringent wedge plate is adhesively bonded to a light incoming plane of the Faraday rotator element, and a light outgoing plane of the Faraday rotator element is adhesively bonded to a non-inclined plane of the second birefringent wedge plate; and the first and second birefringent wedge plates and the Faraday rotator element thus adhesively bonded to each other are disposed in a cylindrical magnet; wherein the crystal axes of those, opposed to each other, of the birefringent wedge plates are offset 90xc2x0 with respect to the common center axis thereof in the light traveling direction; the wedge equal thickness line of the birefringent wedge plates of one of the plurality of optical isolators is offset a specific angle from the wedge equal thickness line of the birefringent wedge plates of another of the plurality of optical isolators; and the optical isolators are adhesively bonded to each other in a state in which a stepped portion provided at an end of the cylindrical magnet of one of the plurality of optical isolators is fitted to a stepped portion provided at an end of the cylindrical magnet of another of the plurality of optical isolators.
In the above multi-stage polarization-independent optical isolator, preferably, the cylindrical magnet is composed of two semi-cylindrical magnets each having an inner recess; and the first birefringent wedge plate, the Faraday rotator element, and the second birefringent wedge plate are fitted in and adhesively bonded to the two inner recesses of the two semi-cylindrical magnets, and simultaneously the two semi-cylindrical magnets are adhesively bonded to each other.
The specific angle is preferably 67.5xc2x0.
The first birefringent wedge plate and/or the second birefringent wedge plate are preferably made from a crystal of lithium niobate.
To achieve the first object, according to a third aspect of the present invention, there is provided a method of producing a polarization-independent optical isolator, including the steps of: preparing two wedge-shaped birefringent crystal bars each having a length of light incoming and outgoing planes, which extends in the wedge equal thickness line direction and is equivalent to lengths of a plurality of optical isolators, and also having an inclined plane as one of the light incoming and outgoing planes and a non-inclined plane as the other of the light outgoing and incoming planes, and a Faraday rotator element crystal bar having a length equivalent to the length of each of the birefringent crystal bars; holding and adhesively bonding light incoming and outgoing planes of the Faraday rotator element crystal bar between the non-inclined planes of the birefringent crystal bars; cutting the integral body of the birefringent crystal bar-Faraday rotator crystal bar-birefringent crystal bar formed at the holding and adhesively bonding step into a plurality of lengths each being equivalent to the length of one optical isolator, to form chips of the birefringent crystal-Faraday rotator crystal-birefringent crystal; and assembling each of the chips in a cylindrical magnet.
In the above method, preferably, the step of assembling each of the chips in the cylindrical magnet comprises a step of fitting the chip in an inner recess of one semi-cylindrical magnet, covering the chip with an inner recess of another semi-cylindrical magnet, and adhesively bonding the chip to the inner recesses and simultaneously adhesively bonding the semi-cylindrical magnets to each other.
To achieve the second object, according to a fourth aspect of the present invention, there is provided a two-stage polarization-independent optical isolator including: a first optical isolator unit including a first birefringent taper plate, a first 45xc2x0 Faraday rotator, and a second birefringent taper plate which are arranged in this order; and a second optical isolator unit including a third birefringent taper plate, a second 45xc2x0 Faraday rotator, and a fourth birefringent taper plate which are arranged in this order; wherein the first optical isolator unit and the second optical isolator unit are arranged in series in a light traveling direction in such a manner that the optical axis of the second birefringent taper plate is offset 90xc2x0 from the optical axis of the third birefringent taper plate, and the inclination direction of the first and second birefringent taper plates is offset 67.5xc2x0 from the inclination direction of the third and fourth birefringent taper plates.
The two-stage polarization-independent optical isolator having the above configuration is capable of enhancing the isolation characteristic and the PMD characteristic, and reducing the excess loss. In this optical isolator, since the crystal axes of the second and third birefringent taper plates are offset 90xc2x0 from each other, of the light emerged from the second birefringent taper plate, a normal light component enters the third birefringent taper plate as an abnormal light component while an abnormal light component enters the third birefringent taper plate as a normal light component. As a result, a difference in transmission time between the normal and abnormal light components caused in the first optical isolator unit is canceled by a difference in transmission time caused in the second optical isolator unit, and consequently, the PMD becomes significantly small. Also, since the inclination direction of the birefringent taper plates of the first optical isolator unit is offset 67.5xc2x0 from the inclination direction of the birefringent taper plates of the second optical isolator unit, it is possible to keep an isolation characteristic comparable to that obtained in the case of 90xc2x0 offset which exhibits the highest isolation characteristic and also make smaller an isolation distance between the normal and abnormal light components of the light when the light is emerged from the second optical isolator unit as compared with the case of 90xc2x0 offset, and hence to make smaller the excess loss caused when the light enters the optical fiber.